Security-printed items such as banknotes, cheques, passports, licences and tickets need to be produced in a manner which allows genuine articles to be authenticated. The security printing industry has seen a wide variety of measures being adopted, ranging from easily-recognisable visual features through discrete visual features to machine-verifiable characteristics. As with the visual features some machine-readable attributes may be relatively readily apparent, such as fluorescent features, while others may be more concealed, requiring specially-made authenticating apparatus.
A security printer is able to select a variety of measures to prevent counterfeiting and forgery and to allow authentication. Any one document will include a range of them, and the choice of those that are actually included in any one document or part of a document presents a formidable obstacle to wrong-doers.
There is a constant need to add to the measures which are employed, particularly those which lend themselves to present-day security printing manufacturing and sorting equipment such as automatic banknote-sorting equipment.
The Raman spectra of chemical compounds have been used for many years as a means of identification. Raman spectra arise when laser light incident upon a sample of the material is scattered: the scattered light includes light of the laser wavelength plus, at much lower intensity, light of additional wavelengths which are characteristic of the compound. The additional light appears at frequencies which are shifted from that of the laser beam by amounts equal to the frequencies of collective vibrations of the atoms in the compound. These frequencies are determined by the masses of the atoms comprising the material and the forces which hold them together. As these are almost always unique for every chemical compound, the Raman spectrum is often used as its fingerprint. In this way, the compound may be identified in various conditions, for example as a crystal, in solution, as a powder and in mixtures with other compounds.
In most respects, conventional Raman scattering spectroscopy provides very similar information to that obtained from infra-red absorption spectroscopy. As the instrumentation is usually considerably more expensive than that for a comparable infra-red apparatus, Raman spectroscopy has usually only been used when infra-red spectroscopy is incapable of providing required information.
Resonance Raman scattering (RRS) occurs when the wavelength of the incident laser beam is equal to, i.e. in resonance with, that of an optical absorption band in the material. The electrons responsible for the absorption are often located on a subset of atoms in the compound, known as the chromophore. Under resonance conditions, the Raman scattered light which is frequency- shifted by the collective vibrations of atoms in the chromophore will be greatly enhanced in intensity.
The intensity of RRS is particularly high for compounds, such as polydiacetylenes, in which the chromophore is a conjugated polymer backbone. The intensities of Raman lines due to collective vibrations of the backbone atoms of a polydiacetylene under resonance conditions can be at least 10.sup.4 times greater than those arising from atomic vibrations in the side-groups.
U.S. Pat. No. 4,125,534 and EP-A-0036899 disclose various carbazolyl polydiacetylenes, their synthesis, and also their use as photoconductors and non-linear optical materials. The polymers which are produced by the given procedures are generally crystalline.